It is desirable to use ultrasound imaging to detect and track interventional devices such as catheters, guide wires, biopsy needles and other devices. Conventional techniques rely on data from other modalities, such as x-ray imaging, to detect the position of the interventional devices. Fluoroscopy, a form of x-ray imaging, provides accurate positional information regarding the interventional device, but it exposes both the patient and the clinician to ionizing radiation. Long-term exposure to ionizing radiation is known to be strongly correlated with negative health effects. Additionally, x-ray imaging is not well-suited for imaging soft tissue, and the knowledge about the precise position of various soft tissue structures is important information for the clinician to have during many interventional procedures. As such, clinicians either have to rely solely on the fluoroscopic images, which may lack vital information about soft tissue structures, or they may need to rely on multiple different imaging modalities. Using multiple imaging modalities either requires specific software to fuse the images together, or the clinician must mentally perform the fusion. In any type of image fusion, there is always the risk that the images may be misregistered, leading to a less precise, or even an ultimately unsuccessful, interventional procedure.
Ultrasound imaging is a non-ionizing modality that excels at imaging soft tissue. Conventional ultrasound techniques are not well-suited for imaging interventional devices, which are typically coherent reflectors. Ultrasound beamforming techniques typically assume that received acoustic reflections come from diffuse scatterers that reflect ultrasound energy in substantially all directions. This assumption proves useful and effective when imaging soft tissue in a patient. However, the underlying physics for coherent reflectors is significantly different than for diffuse scatterers. A coherent reflection is a mirror-like reflection obtained from insonifying a hard level surface with ultrasonic energy. Coherent reflections are common when imaging hard or metal objects, such as catheters and biopsy needles. Ultrasound echoes reflected from a coherent reflector behave according to Snell's law, which means that the angle of incidence is equal to the angle to reflection. Instead of reflecting ultrasound energy in substantially all directions, as is the case with a diffuse reflection, coherent reflections are typically very strong at positions where an angle of reflection of the reflected beam is equal to an angle of incidence. Specular reflections typically generate very little signal at most other locations. As such, ultrasound imaging systems only receive a signal from a coherent reflector if the transducer array is positioned to receive the reflected echo. Many of the echoes reflected from a coherent reflector do not intersect with the transducer array and are therefore not useful for constructing an image of the coherent reflector.
It is desirable to use ultrasound imaging to detect and track the real-time position of coherent reflectors such as catheters, guide wires, needles and other interventional devices. Standard beamforming techniques assume that the reflectors behave as diffuse scatterers. As such, standard beamforming techniques typically sum signals from a plurality of channels in order to form an ultrasound image. While this approach has proven very effective for soft tissue and other circumstances where the imaged material behaves as a diffuse scatterer, it is ineffective when imaging coherent reflectors. The coherent reflector will not contribute significant signal to elements when the reflected beam is away from the probe. And, if a conventional beamforming technique is applied to ultrasound data including a coherent reflection, the contributions of the coherent reflector tend to get minimized during the summing process. Therefore, conventional beamforming techniques are not effective for imaging coherent reflectors.
Conventional systems may use an external tracking system, such as an electromagnetic tracking system or an optical tracking system, to determine the position and orientation of an interventional device in real-time. However, using an external tracking system adds additional expense and complexity to the entire system. Additionally, the ultrasound system is required to be configured to interface with the tracking system if data showing the location and/or the trajectory of the interventional device is to be displayed in real-time.
It is also known to use a needle guide that acts as a fixture keeping the probe in a constant relative position with respect to a needle being imaged. While this technique is effective for imaging needles, the needle guide combined with the probe and the needle is bulkier and potentially more difficult to maneuver than a stand-alone needle. Additionally, this technique does not work to track other types of interventional devices that are disposed completely within the patient. In ultrasound biopsy, the needle guide is typically used to guide the needle into the tissue so that the needle is orthogonal to the transmit beam. In a 2D B-mode scan it is difficult for the operator to determine if the tip of the needle is inside the image frame. Determining if the tip of the needle is inside the 2D image typically involves a fair deal of trial- and error by inserting and retracting the needle. Conventional techniques are currently not well-suited for determining if the needle tip is inside of the 2D image frame. Conventional techniques rely completely upon operator experience to ensure that the needle tip is visible within the 2D image frame. As such, it is desirable to have an automated method to adjust the position of the 2D image frame so that the tip is visible in the 2D image frame.
While performing an interventional procedure, it is often very important to know the position and orientation of the interventional device, such as a needle, a catheter, or any other type of interventional device both to achieve the desired clinical outcome and to avoid potentially damaging surrounding tissue. For these and other reasons an improved method and ultrasound imaging system detecting and tracking coherent reflectors, such as catheter, guidewires, and biopsy needles is desired.